About 275 patients are already sporting the metal-heavy implants from the TC-PLUS, VKS and RT-PLUS ranges, and the company is trying to track down another 300 which have been "supplied to customers", mainly in Europe.

Smith & Nephew attributes the excess of iron to a "production error at the supplier's factory", but insists it isn't expected to have any serious health repercussions for those already enjoying new knees. Rather, its concern is that the implants might not prove as durable as they otherwise should.

The company said in a statement: "There have been no reports to date of any patient problems resulting from the use of these implants. Smith & Nephew is carrying out laboratory testing to determine if the implants' material properties or durability have been affected."

A spokeswoman admitted that whipping the implants out might cause "more distress and worry" than leaving them be, adding: "What we are advising patients to do, if they know they have had one of these implants, is to speak to their surgeon." ®